1. Field of the Invention
The present invention relates to an SOI high-voltage switch with a FET structure in which a drift zone of one conductivity type is provided between a gate electrode and a drain electrode in the drain region.
2. Background of the Invention
SOI structures are particularly suited for integrated circuits operated with high voltage (HV-IC). This is because, under certain circumstances, such HV-ICs can serve as full bridges with driver functions on a chip. A precondition to this, however, is that lateral SOI structures which are required for such applications have relatively high-impedance drift zones in their drain region.
This problem already has been known for a sum time in the case of vertical structures in order, for example in the context of switching off GTO thyristors, to prevent a decrease in the gate turn-off current as the voltage rises. In addition to field rings, freely floating regions of the second conductivity type are inserted into the region of space charge zones in a substrate of the first conductivity type, the doping of these freely floating regions being performed in such a way that doping reversal of the substrate is always ensured (cf. EP 0 344 514 B1). Moreover, in the case of an SOI thin-film transistor, it is known to provide a drift zone with a linear doping profile in order to improve the dielectric strength (cf. EP 0 497 427 B1).
The first-mentioned measure, that of the introduction of a freely floating region, does not take account of the specific requirements of lateral arrangements and does not give attention to the configuration of the edge of the FET structure. The second measure, that of the provision of a linear doping profile in the region of the drift zone, is relatively complicated and requires adaptation to the extent of the drift zone.
Taking such prior art as a starting point, an object of the present invention is to provide an SOI high-voltage switch which can be produced in a simple manner for practically any desired lateral extent and which has a high dielectric strength.
In the case of an SOI high-voltage switch of the type mentioned in the introduction, this object is achieved according to the invention by virtue of the fact that pillar-like trenches are incorporated in the drift zone and filled with semiconductor material of the other conductivity type.
These trenches, which preferably are filled with polycrystalline silicon which is doped with dopant of the other conductivity type, are arranged, for example, in rows in a grid-like manner. The distance between the rows running in the direction perpendicular to the connection direction between drain and source is approximately 3 to 30 xcexcm, preferably 6 to 10 xcexcm, in the case of n-type doping of the drift zone of approximately 2xc3x971015 cmxe2x88x923. The distance between the trenches in a row may be approximately 2 to 5 xcexcm if the trenches, which may have a circular or rectangular cross section, have cross-sectional dimensions of about 0.1 to 3 xcexcm, in particular 1 to 2 xcexcm.
In order to increase the dielectric strength further, field plates additionally may be provided in the edge region on the surface, which field plates are to be connected to gate or source of the SOI high-voltage switch.
The individual trenches are introduced all at once into the drift zone, for example by etching, and subsequently are filled with p-doped polycrystalline silicon, if the drift zone is n-doped. In the event of a thermal treatment, the dopant penetrates out from the polycrystalline silicon, such that there are a number of xe2x80x9cplugxe2x80x9d or xe2x80x9cdumplingxe2x80x9d like sources of p-type dopant, for example boron, in the n-conducting drift zone. It is therefor, possible to dope the drift zone more highly without risking a reduction in the dielectric strength.
The trenches are specifically arranged in such a way that those regions of the drift zone which lie between them, so-called intermediate zones, are depleted of charge carriers beforehand, in the event of positive drain voltage being applied to the n+-conducting drain zone or the n-conducting drift zone, before a breakdown can occur between the p-conducting trench and the n-conducting surroundings of the drift zone.
The principle of the present invention, that is the introduction of individual trenches in a drift zone of one conductivity type wherein the trenches are arranged in a grid-like manner and filled with semiconductor material of the other conductivity type, can be applied not only to SOI-FETs but also, for example, to SOI-IGBTs (insulating gate bipolar transistors), if a high current-carrying capacity is sought even at relatively high voltages.